Activation of Guinea Pig Atrial Muscarinic K+Channels by Nucleoside Triphosphates in the Absence of Acetylcholine

Effects of the novel amiodarone-like compound SAR114646A on cardiac ion channels and ventricular arrhythmias in rats

Amiodarone is the "gold standard" for current antiarrhythmic therapy because it combines efficacy with good hemodynamic and electrophysiological tolerance. Amiodarone is effective against both atrial and ventricular arrhythmias by intravenous (i.v.) or oral route. However, the i.v. formulation has limitations. Therefore, we identified SAR114646A, an amiodarone-like antiarrhythmic agent with good aqueous solubility suitable for i.v. application. Patch-clamp experiments were performed with isolated cardiomyocytes from guinea pigs and rats. In guinea pig ventricular cardiomyocytes, the fast Na(+) channel and the L-type Ca(2+) channels were blocked by SAR114646A with half-maximal concentrations (IC(50)) of 2.0 and 1.1 μM, respectively. The tail current of the fast activating rectifying potassium channel I(Kr) was blocked with an IC(50) value of 0.6 μM, whereas the IC(50) values for inhibition of the I(Ks) and I(K1) channels was >10 μM. ATP-sensitive K(+) channels were evoked by application of the channel opener rilmakalim (3 μM). SAR114646A blocked this current with an IC(50) value of 2.8 μM. In guinea pig atrial cardiomyocytes, carbachol (1 μM) was used to activate the I(KACh) and SAR114646A inhibited this current with IC(50) of 36.5 nM. The transient outward current I(to) and the sustained current I(sus) were investigated in rat ventricular myocytes. SAR114646A blocked these currents with IC(50) of 1.8 and 1.2 μM, respectively. When expressed in Chinese hamster ovary cells, the currents hKv1.5 and hHCN4 were inhibited with IC(50) values of 1.1 and 0.4 μM, respectively. Micropuncture experiments in isolated rabbit left atria revealed that SAR114646A prolonged the 50% repolarization significantly at 3 and 10 μM. In guinea pig papillary muscle, the APD at 90% of repolarization was slightly prolonged at 3 and 10 μM. SAR114646A demonstrates antiarrhythmic activity in anaesthetised rats, subjected to 5 min ischemia followed by 10 min reperfusion, where 1 mg/kg of SAR114646A applied as i.v. bolus 5 min prior to ischemia, decreased mortality to 0% compared to 80% under control conditions. In conclusion, SAR114646A is a multichannel blocker with improved water solubility, compared to amiodarone. In contrast to amiodarone, SAR114646A also blocks the K(+) channels I(to) and I(sus). A potent antiarrhythmic effect as observed in rats can also be expected in other animal models.

Characterization of a hyperpolarization-activated time-dependent potassium current in canine cardiomyocytes from pulmonary vein myocardial sleeves and left atrium

Cardiomyocytes from the pulmonary vein sleeves (PVs) are known to play an important role in atrial fibrillation. PVs have been shown to exhibit time-dependent hyperpolarization-induced inward currents of uncertain nature. We observed a time-dependent K(+) current upon hyperpolarization of PV and left atrial (LA) cardiomyocytes (I(KH)) and characterized its biophysical and pharmacological properties. The activation time constant was weakly voltage dependent, ranging from 386 +/- 14 to 427 +/- 37 ms between -120 and -90 mV, and the half-activation voltage averaged -93 +/- 4 mV. I(KH) was larger in PV than LA cells (e.g. at -120 mV: -2.8 +/- 0.3 versus-1.9 +/- 0.2 pA pF(-1), respectively, P < 0.01). The reversal potential was approximately -84 mV with 5.4 mm[K(+)](o) and changed by 55.7 +/- 2.4 mV per decade [K(+)](o) change. I(KH) was exquisitely Ba(2+) sensitive, with a 50% inhibitory concentration (IC(50)) of 2.0 +/- 0.3 microm (versus 76.0 +/- 17.9 microm for instantaneous inward-rectifier current, P < 0.01), and showed similar Cs(+) sensitivity to instantaneous current. I(KH) was potently blocked by tertiapin-Q, a selective Kir3-subunit channel blocker (IC(50) 10.0 +/- 2.1 nm), was unaffected by atropine and was significantly increased by isoproterenol (isoprenaline), carbachol and the non-hydrolysable guanosine triphosphate analogue GTPgammaS. I(KH) activation by carbachol required GTP in the pipette and was prevented by pertussis toxin pretreatment. Tertiapin-Q delayed repolarization in atropine-exposed multicellular atrial preparations studied with standard microelectrodes (action potential duration pre- versus post-tertiapin-Q: 190.4 +/- 4.3 versus 234.2 +/- 9.9 ms, PV; 202.6 +/- 2.6 versus 242.7 +/- 6.2 ms, LA; 2 Hz, P < 0.05 each). Seven-day atrial tachypacing significantly increased I(KH) (e.g. at -120 mV in PV: from -2.8 +/- 0.3 to -4.5 +/- 0.5 pA pF(-1), P < 0.01). We conclude that I(KH) is a time-dependent, hyperpolarization-activated K(+) current that likely involves Kir3 subunits and appears to play a significant role in atrial physiology.

On the role of phosphatase in regulation of cardiac L-type calcium current by cyclic GMP

Does cGMP, via protein kinase G, inhibit cAMP-stimulated Ca(2+) current (I(Ca(L))) in mammalian ventricular myocytes by phosphorylating the calcium channel at a site different from that acted on by cAMP or by dephosphorylating the calcium channel through phosphatase(s)? We tested these possibilities in guinea pig ventricular myocytes superfused with Tyrode's solution (35 degrees C) and dialyzed with adenosine 5'-O-(3-thiotriphosphate) ([ATPgammaS](pip)). ATPgammaS is a kinase substrate but thiophosphorylated proteins are not phosphatase substrates. With 5 mM [ATPgammaS](pip), I(Ca(L)) increased gradually over 20 to 25 min and then rapidly in the presence of 3-isobutyl-1-methylxanthine. 8-Bromo-cGMP (8-Br-cGMP; 1 mM) did not inhibit I(Ca(L)) significantly (-3 +/- 11.8%, n = 21) in contrast to results with ATP dialysis (). Similar results were obtained with 0.1 mM carbachol (CCh). I(Ca(L)) increased after longer dialysis (>/=40 min) with ATPgammaS; again, 8-Br-cGMP had no effect. Also, isoproterenol (ISO) did not stimulate and CCh, alone or in the presence of ISO, did not inhibit I(Ca(L)). Block of CCh effect by ATPgammaS, although consistent with cGMP action in muscarinic inhibition, could be explained by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) formation from ATPgammaS via nucleoside diphosphate kinase. GTPgammaS uncouples muscarinic and beta-adrenoceptors from intracellular effectors. Failure of 8-Br-cGMP to reduce I(Ca(L)) irreversibly excludes calcium channel phosphorylation as an inhibitory mechanism. We propose that cGMP inhibits I(Ca(L)) by activating phosphatase(s) in guinea pig ventricular myocytes.

ATP-dependent desensitization of the muscarinic K+ channel in rat atrial cells

1. Fast desensitization of the muscarinic K+ channel has been studied in excised patches from rat atrial cells. 2. In inside-out patches, ACh was present in the pipette and GTP was applied via the bath to activate the channel. In outside-out patches, GTP was present in the pipette and ACh was applied via the bath to activate the channel. In both cases, during a 30 s exposure to GTP or ACh there was a decline in channel activity as a result of fast desensitization if ATP was present. 3. In inside-out patches, fast desensitization was still observed if the muscarinic ACh receptor was bypassed and the channel was activated by GTP gamma S. This suggests that fast desensitization is a result of a modification of the channel (or the connecting G protein) and not the receptor. 4. In both inside-out and outside-out patches, channel activity was depressed and fast desensitization was reduced or absent, if ATP was not present. 5. The non-hydrolysable analogue of ATP, AMP-PNP, did not substitute for ATP in its effects on the channel. 6. The results are consistent with the hypothesis that fast desensitization of the muscarinic K+ channel is the result of a dephosphorylation of the channel.

Acetylcholine-mediated K+ channel activity in guinea-pig atrial cells is supported by nucleoside diphosphate kinase

We studied the role of nucleoside diphosphate kinase (NDPK) in acetylcholine-mediated muscarinic K+ channel activation in inside-out patches of guinea-pig atrial cells. NDPK-catalysed activation of the muscarinic K+ channels by adenosine triphosphate-Mg2+ (ATP-Mg2+) is not prevented by occupation of the muscarinic receptor [by acetylcholine (ACh) or atropine], nor by uncoupling of the receptor from the G protein by pertussis-toxin-catalysed adenosine diphosphate (ADP)-ribosylation of GK. In the presence of ACh, addition of 0.1 mM guanosine triphosphate (GTP) after activation of the channels by 4 mM ATP alone resulted in a moderate increase of channel activity (in contrast to block in the absence of ACh): NDPK-mediated direct transphosphorylation is uncoupled by the G nucleotide but agonist-induced guanosine diphosphate (GDP)-to-GTP exchange takes over activation of the channels. Moreover, ACh-dependent channel stimulation was possible in inside-out patches while ATP and GDP were present in the bathing solution (in contrast to the complete absence of channel activation in the absence of ACh). This indicates that NDPK synthesizes sufficient GTP to support channel activation by exchange. Hence, it is postulated that the main functional role of NDPK under physiological conditions is to provide a local supply of GTP (using GDP and ATP) in the immediate vicinity of the G protein, thereby maintaining a high local GTP/GDP ratio and ensuring adequate receptor-mediated regulation of muscarinic K+ channel activity.